Episodes of chip formation in micro-to-nanoscale cutting of Inconel 625

Abstract

Nickel-chromium superalloys (Inconel 625) are used for various conventional/macro-sized products as well as for micro/nano applications. However, machining of these ‘difficult-to-cut’ materials is challenging due to their extreme toughness, high temperature resistance and continuous work hardening. Thus, the tool-based cutting processes require improvements for micro/nano applications as the chip formation mechanisms in micro-to-nanoscale are not yet understood clearly. Hence, in this study, material removal mechanisms have been comprehensively analysed in ultra-precision machining of Inconel 625 at three distinct regimes of micrometer, sub-micrometer and nanometer level. The physical responses of material were analogized to the fundamental mechanisms of cutting, ploughing, sliding and burnishing by the change of removal scale. In microscale cutting, continuous chips with lamella-like structure were noticed akin to conventional cutting. Sub-microscale machining delivers significant variations of the material removal episodes as chip perforation, bifurcation, fragmentation and structural heterogeneity as the tool advances from grain to grain, thus, influencing the change of chip thickness, striations and machining forces. The fluctuation of force profiles noticed due to the material pile-up effect influencing the overall coefficient of friction (COF). Nanoscale removal persisted from the grain cutting phenomena with the transformation of material cutting to material deformation in sliding-like behaviour. Prevalence of damage on the machined surface, induced by tensile stress, diminishes from micro to sub-micrometer removal lengths. Nanoscale machined surface, affected by the high value of negative effective angle of rake and compressive stress, correlated with the slide burnishing (SB) phenomena for improved surface finishing. This study, thus, provides a fundamental basis on various aspects of tool-based processes of Ni-Cr superalloy in terms of material behaviour, tool-work interaction, chip formation and surface generation in micro-to-nanoscale operations.